This invention relates to a process for the extraction of oil from oil-containing vegetable materials, in particular oil seed, such as soybeans or rape seed.
It is known to extract oil-containing solids by use of liquid solvents, such as hexane, after a disintegration treatment. In the case of seeds which are particularly rich in oil it is suitable to effect a previous mechanical oil removal by pressing. A disadvantage of this type of extraction is the removal of the liquid solvent from the residue and the extract. This removal is costly and incomplete, leading to solvent losses. Furthermore, it was found that hexane can extract, in addition to glycerides, many substances which accompany the fat, such as phosphatides and which have to be removed separately.
The separation of the residue and the extractant is improved in the so-called gas-extraction in which a super-critical gas is used as the extractant, and the separation of the extracted oil is achieved by a modification of density by pressure relief and/or temperature increase. In the gas-extraction, carbon dioxide or ethene may be used in super-critical state. Although carbon dioxide is physiologically acceptable and non-burnable it has a relatively low solubility for glycerides so that high pressures, such as about 700 bar, have had to be used to obtain a sufficiently short extraction time. Although ethane is a better solvent for glycerides than carbon dioxide, said extractant is also not satisfying. When ethane or any other gaseous extractant is used, the separation of the extracted oil has to be achieved by an essential reduction of its density for which purpose a temperature increase or a decrease of pressure are suitable. A pressure relief is much more effective, all the more, because the temperature stability of the oil to be obtained is limited. However, in that process, the pressure of the extractant has to be reduced substantially (in many cases at least to half the value of the starting pressure), whereupon the extractant has to be recompressed to the starting pressure.